1. Field of Invention
This invention relates generally to warning devices, and especially to fire or smoke detectors employing sensing or detecting devices responsive to changes in the ambient atmosphere produced by the products of combustion. The invention particularly provides an electronically compensated, rate-of-change sensing device which is non-responsive to slow changes in natural conditions, such as changes in the relative humidity or the pressure of the ambient air, while maintaining the device fully responsive to rapid changes, such as occur in the ambient air due to combustion.
2. Prior Art
Several techniques for compensating rate-of-change sensing devices are known. One such system utilizes a pair of similar sensing devices connected in a bridge circuit. One of the sensing devices is constructed to allow a free flow of air thereto, whereas the other sensing device is shielded to restrict or prevent the flow of air thereto. In the event of a slowly occurring change in an atmospheric condition, both sensors respond similarly so that the changes detected by the sensors effectively cancel each other. In the event of a rapidly occurring change, as in the event of fire, the unshielded sensor responds more rapidly than the shielded sensor, and a warning signal will be provided.
Another compensating system, described in U.S. Pat. No. 3,548,205, issued Dec. 15, 1970, to Wilbur L. Ogden, utilizes a MOSFET transistor connected to the sensing device. The transistor is self-biased and includes a filter capacitor which allows the conductivity of the MOSFET to vary to compensate for slowly occurring changes in the impedance of the detecting device, but prevents compensation of rapidly occurring changes.
Though both these prior techniques provide ways to compensate warning devices for slowly changing parameters sensed by the sensor device, they require the use of individually selected and carefully matched components. The particular sensing device chosen must have electrical characteristics that are compatible with the characteristics of the particular components of the amplification circuitry connected to that sensing device. As can be appreciated selecting and matching the various components for assembly on a production basis is difficult, time consuming and requires considerable skill.